Hierarchical category classification scheme using multiple sets of fully-connected networks with a CNN based integrated circuit as feature extractor

ABSTRACT

CNN based integrated circuit is configured with a set of pre-trained filter coefficients or weights as a feature extractor of an input data. Multiple fully-connected networks (FCNs) are trained for use in a hierarchical category classification scheme. Each FCN is capable of classifying the input data via the extracted features in a specific level of the hierarchical category classification scheme. First, a root level FCN is used for classifying the input data among a set of top level categories. Then, a relevant next level FCN is used in conjunction with the same extracted features for further classifying the input data among a set of subcategories to the most probable category identified using the previous level FCN. Hierarchical category classification scheme continues for further detailed subcategories if desired.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part (CIP) to a co-pending U.S. patent application Ser. No. 15/709,220 for “Natural Language Processing Using A CNN Based Integrated Circuit” filed on Sep. 19, 2017, which is a CIP to a co-pending U.S. patent application Ser. No. 15/694,711 for “Natural Language Processing Via A Two-dimensional Symbol Having Multiple Ideograms Contained Therein” filed on Sep. 1, 2017, which is a CIP to a co-pending U.S. patent application Ser. No. 15/683,723 for “Two-dimensional Symbols For Facilitating Machine Learning Of Combined Meaning Of Multiple Ideograms Contained Therein” filed on Aug. 22, 2017, which claims priority from a U.S. Provisional Patent Application Ser. No. 62/541,081, entitled “Two-dimensional Symbol For Facilitating Machine Learning Of Natural Languages Having Logosyllabic Characters” filed on Aug. 3, 2017.

This application is a continuation-in-part (CIP) to a co-pending U.S. patent application Ser. No. 15/289,726 for “Digital Integrated Circuit For Extracting Features Out Of An Input Image Based On Cellular Neural Networks” filed on Oct. 10, 2016.

This application claims priority from a U.S. Provisional Patent Application Ser. No. 62/548,905, entitled “Hierarchical Category Classification Scheme Using Multiple Sets of Fully-Connected Networks With A CNN Based Integrated Circuit As Feature Extractor” filed on Nov. 12, 2017.

All of which are hereby incorporated by reference in their entirety for all purposes.

FIELD

The invention generally relates to the field of integrated circuit and more particularly to a digital integrated circuit for extracting features out of an input image based on Cellular Neural Networks.

BACKGROUND

Cellular Neural Networks or Cellular Nonlinear Networks (CNN) have been applied to many different fields and problems including, but limited to, image processing since 1988. However, most of the prior art CNN approaches are either based on software solutions (e.g., Convolutional Neural Networks, Recurrent Neural Networks, etc.) or based on hardware that are designed for other purposes (e.g., graphic processing, general computation, etc.). As a result, CNN prior approaches are too slow in term of computational speed and/or too expensive thereby impractical for processing large amount of imagery data. The imagery data can be from any two-dimensional data (e.g., still photo, picture, a frame of a video stream, converted form of voice data, etc.)

SUMMARY

This section is for the purpose of summarizing some aspects of the invention and to briefly introduce some preferred embodiments. Simplifications or omissions in this section as well as in the abstract and the title herein may be made to avoid obscuring the purpose of the section. Such simplifications or omissions are not intended to limit the scope of the invention.

A hierarchical category classification scheme in conjunction with a pre-configured CNN based integrated circuit is disclosed. According to one aspect of the invention, CNN based integrated circuit is configured with a set of pre-trained filter coefficients or weights as a feature extractor of an input data. Multiple fully-connected networks (FCNs) are trained for use in a hierarchical category classification scheme. Each FCN is capable of classifying the input data via the extracted features in a specific level of the hierarchical category classification scheme. First, a root level FCN is used for classifying the input data among a set of top level categories. Then, a relevant next level FCN is used in conjunction with the same extracted features for further classifying the input data among a set of subcategories to the most probable category identified using the previous level FCN. Hierarchical category classification scheme continues for further detailed subcategories if desired.

According to another aspect, improvement to computational speed of fully-connected networks is to remove neurons or cells of subsequent layers of a FCN that do not contribute to classification of the categories. One of the techniques is to identify those neuron or cells having input value of zero for more than a threshold percentage of all data in a desired training dataset in the particular domain.

According to yet another aspect, improvement to computational speed of fully-connected networks is to remove any unnecessary connections between neurons or cells of subsequent layers of a FCN. One of the techniques is to identify those connections with the numerical value of corresponding weights or coefficients equal to zero or less than a predefined value,

One advantage of the invention is to simplify the training, convolutional layers are pre-trained and stay the same for each different FCN. Only layers in each FCN need to be trained instead of entire model. This saves time and labeled dataset size for training as only relatively few layers need to be adjusted.

Another advantage of the invention is to be able to classify more detailed categories due to hierarchical category classification scheme, because relatively few categories are used in each level of the hierarchical category classification scheme.

Objects, features, and advantages of the invention will become apparent upon examining the following detailed description of an embodiment thereof, taken in conjunction with the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the invention will be better understood with regard to the following description, appended claims, and accompanying drawings as follows:

FIGS. 1A-1B are block diagrams illustrating an example integrated circuit designed for extracting features from input imagery data in accordance with one embodiment of the invention;

FIG. 2 is a function block diagram showing an example controller configured for controlling operations of one or more CNN processing engines according to an embodiment of the invention;

FIG. 3 is a diagram showing an example CNN processing engine in accordance with one embodiment of the invention;

FIG. 4 is a diagram showing M×M pixel locations within a (M+2)-pixel by (M+2)-pixel region, according to an embodiment of the invention;

FIGS. 5A-5C are diagrams showing three example pixel locations, according to an embodiment of the invention;

FIG. 6 is a diagram illustrating an example data arrangement for performing 3×3 convolutions at a pixel location, according to one embodiment of the invention;

FIG. 7 is a function block diagram illustrating an example circuitry for performing 3×3 convolutions at a pixel location, according to one embodiment of the invention;

FIG. 8 is a diagram showing an example rectification according to an embodiment of the invention;

FIGS. 9A-9B are diagrams showing two example 2×2 pooling operations according to an embodiment of the invention;

FIG. 10 is a diagram illustrating a 2×2 pooling operation reduces M-pixel by M-pixel block to a (M/2)-pixel by (M/2)-pixel block in accordance with one embodiment of the invention;

FIGS. 11A-11C are diagrams illustrating examples of M-pixel by M-pixel blocks and corresponding (M+2)-pixel by (M+2)-pixel region in an input image, according to one embodiment of the invention;

FIG. 12 is a diagram illustrating an example of a first set of memory buffers for storing received imagery data in accordance with an embodiment of the invention;

FIG. 13A is a diagram showing two operational modes of an example second set of memory buffers for storing filter coefficients in accordance with an embodiment of the invention;

FIG. 13B is a diagram showing example storage schemes of filter coefficients in the second set of memory buffers, according to an embodiment of the invention;

FIG. 14 is a diagram showing a plurality of CNN processing engines connected as a loop via an example clock-skew circuit in accordance of an embodiment of the invention;

FIG. 15 is a schematic diagram showing an example image processing technique based on convolutional neural networks in accordance with an embodiment of the invention;

FIG. 16 is a flowchart illustrating an example process of achieving a trained convolutional neural networks model having bi-valued 3×3 filter kernels in accordance with an embodiment of the invention;

FIG. 17 is a diagram showing an example filter kernel conversion scheme in accordance with the invention;

FIG. 18 is a diagram showing an example data conversion scheme;

FIG. 19A is a schematic diagram showing data flow of an example feature extractor in accordance with one embodiment of the invention;

FIGS. 19B-19D are schematic diagrams showing an example hierarchical classification scheme with multiple fully-connected networks in accordance with one embodiment of the invention;

FIG. 20 is a schematic diagram showing example connection patterns an example FCN in accordance with one embodiment of the invention;

FIGS. 21A-21B are schematic diagrams showing a first example computation reduction scheme in FCN in accordance with one embodiment of the invention;

FIGS. 22A-22B are schematic diagrams showing a second example computation reduction scheme in FCN in accordance with one embodiment of the invention; and

FIG. 23 is a flowchart showing an example process of computation reduction scheme in a FCN according to an embodiment of the invention.

DETAILED DESCRIPTIONS

In the following description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will become obvious to those skilled in the art that the invention may be practiced without these specific details. The descriptions and representations herein are the common means used by those experienced or skilled in the art to most effectively convey the substance of their work to others skilled in the art. In other instances, well-known methods, procedures, and components have not been described in detail to avoid unnecessarily obscuring aspects of the invention.

Reference herein to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments. Further, the order of blocks in process flowcharts or diagrams or circuits representing one or more embodiments of the invention do not inherently indicate any particular order nor imply any limitations in the invention. Used herein, the terms “top”, “bottom”, “right” and “left” are intended to provide relative positions for the purposes of description, and are not intended to designate an absolute frame of reference

Embodiments of the invention are discussed herein with reference to FIGS. 1A-23. However, those skilled in the art will readily appreciate that the detailed description given herein with respect to these figures is for explanatory purposes as the invention extends beyond these limited embodiments.

Referring first to FIG. 1A, it is shown a block diagram illustrating an example digital integrated circuit (IC) 100 for extracting features out of an input image in accordance with one embodiment of the invention.

The integrated circuit 100 is implemented as a digital semi-conductor chip and contains a CNN processing engine controller 110, and one or more neural networks (CNN) processing engines 102 operatively coupled to at least one input/output (I/O) data bus 120. Controller 110 is configured to control various operations of the CNN processing engines 102 for extracting features out of an input image based on an image processing technique by performing multiple layers of 3×3 convolutions with rectifications or other nonlinear operations (e.g., sigmoid function), and 2×2 pooling operations. To perform 3×3 convolutions requires imagery data in digital form and corresponding filter coefficients, which are supplied to the CNN processing engine 102 via input/output data bus 120. It is well known that digital semi-conductor chip contains logic gates, multiplexers, register files, memories, state machines, etc.

According to one embodiment, the digital integrated circuit 100 is extendable and scalable. For example, multiple copy of the digital integrated circuit 100 can be implemented on one semiconductor chip.

All of the CNN processing engines are identical. For illustration simplicity, only few (i.e., CNN processing engines 122 a-122 h, 132 a-132 h) are shown in FIG. 1B. The invention sets no limit to the number of CNN processing engines on a digital semi-conductor chip.

Each CNN processing engine 122 a-122 h, 132 a-132 h contains a CNN processing block 124, a first set of memory buffers 126 and a second set of memory buffers 128. The first set of memory buffers 126 is configured for receiving imagery data and for supplying the already received imagery data to the CNN processing block 124. The second set of memory buffers 128 is configured for storing filter coefficients and for supplying the already received filter coefficients to the CNN processing block 124. In general, the number of CNN processing engines on a chip is 2^(n), where n is an integer (i.e., 0, 1, 2, 3, . . . ). As shown in FIG. 1B, CNN processing engines 122 a-122 h are operatively coupled to a first input/output data bus 130 a while CNN processing engines 132 a-132 h are operatively coupled to a second input/output data bus 130 b. Each input/output data bus 130 a-130 b is configured for independently transmitting data (i.e., imagery data and filter coefficients). In one embodiment, the first and the second sets of memory buffers comprise random access memory (RAM). Each of the first and the second sets are logically defined. In other words, respective sizes of the first and the second sets can be reconfigured to accommodate respective amounts of imagery data and filter coefficients.

The first and the second I/O data bus 130 a-130 b are shown here to connect the CNN processing engines 122 a-122 h, 132 a-132 h in a sequential scheme. In another embodiment, the at least one I/O data bus may have different connection scheme to the CNN processing engines to accomplish the same purpose of parallel data input and output for improving performance.

FIG. 2 is a diagram showing an example controller 200 for controlling various operations of at least one CNN processing engine configured on the integrated circuit. Controller 200 comprises circuitry to control imagery data loading control 212, filter coefficients loading control 214, imagery data output control 216, and image processing operations control 218. Controller 200 further includes register files 220 for storing the specific configuration (e.g., number of CNN processing engines, number of input/output data bus, etc.) in the integrated circuit.

Image data loading control 212 controls loading of imagery data to respective CNN processing engines via the corresponding I/O data bus. Filter coefficients loading control 214 controls loading of filter coefficients to respective CNN processing engines via corresponding I/O data bus. Imagery data output control 216 controls output of the imagery data from respective CNN processing engines via corresponding I/O data bus. Image processing operations control 218 controls various operations such as convolutions, rectifications and pooling operations which can be defined by user of the integrated circuit via a set of user defined directives (e.g., file contains a series of operations such as convolution, rectification, pooling, etc.).

More details of a CNN processing engine 302 are shown in FIG. 3. A CNN processing block 304 contains digital circuitry that simultaneously obtains M×M convolution operations results by performing 3×3 convolutions at M×M pixel locations using imagery data of a (M+2)-pixel by (M+2)-pixel region and corresponding filter coefficients from the respective memory buffers. The (M+2)-pixel by (M+2)-pixel region is formed with the M×M pixel locations as an M-pixel by M-pixel central portion plus a one-pixel border surrounding the central portion. M is a positive integer. In one embodiment, M equals to 14 and therefore, (M+2) equals to 16, M×M equals to 14×14=196, and M/2 equals 7.

FIG. 4 is a diagram showing a diagram representing (M+2)-pixel by (M+2)-pixel region 410 with a central portion of M×M pixel locations 420 used in the CNN processing engine 302.

Imagery data may represent characteristics of a pixel in the input image (e.g., one of the color (e.g., RGB (red, green, blue)) values of the pixel, or distance between pixel and observing location). Generally, the value of the RGB is an integer between 0 and 255. Values of filter coefficients are floating point integer numbers that can be either positive or negative.

In order to achieve faster computations, few computational performance improvement techniques have been used and implemented in the CNN processing block 304. In one embodiment, representation of imagery data uses as few bits as practical (e.g., 5-bit representation). In another embodiment, each filter coefficient is represented as an integer with a radix point. Similarly, the integer representing the filter coefficient uses as few bits as practical (e.g., 12-bit representation). As a result, 3×3 convolutions can then be performed using fixed-point arithmetic for faster computations.

Each 3×3 convolution produces one convolution operations result, Out(m, n), based on the following formula:

$\begin{matrix} {{{Out}\left( {m,n} \right)} = {{\sum\limits_{{1 \leq i},{j \leq 3}}\;{{{In}\left( {m,n,i,j} \right)} \times {C\left( {i,j} \right)}}} - b}} & (1) \end{matrix}$ where:

-   -   m, n are corresponding row and column numbers for identifying         which imagery data (pixel) within the (M+2)-pixel by (M+2)-pixel         region the convolution is performed;     -   In(m,n,i,j) is a 3-pixel by 3-pixel area centered at pixel         location (m, n) within the region;     -   C(i, j) represents one of the nine weight coefficients C(3×3),         each corresponds to one of the 3-pixel by 3-pixel area;     -   b represents an offset coefficient; and     -   i, j are indices of weight coefficients C(i, j).

Each CNN processing block 304 produces M×M convolution operations results simultaneously and, all CNN processing engines perform simultaneous operations.

FIGS. 5A-5C show three different examples of the M×M pixel locations. The first pixel location 531 shown in FIG. 5A is in the center of a 3-pixel by 3-pixel area within the (M+2)-pixel by (M+2)-pixel region at the upper left corner. The second pixel location 532 shown in FIG. 5B is one pixel data shift to the right of the first pixel location 531. The third pixel location 533 shown in FIG. 5C is a typical example pixel location. M×M pixel locations contain multiple overlapping 3-pixel by 3-pixel areas within the (M+2)-pixel by (M+2)-pixel region.

To perform 3×3 convolutions at each sampling location, an example data arrangement is shown in FIG. 6. Imagery data (i.e., In(3×3)) and filter coefficients (i.e., weight coefficients C(3×3) and an offset coefficient b) are fed into an example CNN 3×3 circuitry 600. After 3×3 convolutions operation in accordance with Formula (1), one output result (i.e., Out(1×1)) is produced. At each sampling location, the imagery data In(3×3) is centered at pixel coordinates (m, n) 605 with eight immediate neighbor pixels 601-604, 606-609.

FIG. 7 is a function diagram showing an example CNN 3×3 circuitry 700 for performing 3×3 convolutions at each pixel location. The circuitry 700 contains at least adder 721, multiplier 722, shifter 723, rectifier 724 and pooling operator 725. In a digital semi-conductor implementation, all of these can be achieved with logic gates and multiplexers, which are generated using well-known methods (e.g., hardware description language such as Verilog, etc.). Adder 721 and multiplier 722 are used for addition and multiplication operations. Shifter 723 is for shifting the output result in accordance with fixed-point arithmetic involved in the 3×3 convolutions. Rectifier 724 is for setting negative output results to zero. Pooling operator 725 is for performing 2×2 pooling operations.

Imagery data are stored in a first set of memory buffers 306, while filter coefficients are stored in a second set of memory buffers 308. Both imagery data and filter coefficients are fed to the CNN block 304 at each clock of the digital integrated circuit. Filter coefficients (i.e., C(3×3) and b) are fed into the CNN processing block 304 directly from the second set of memory buffers 308. However, imagery data are fed into the CNN processing block 304 via a multiplexer MUX 305 from the first set of memory buffers 306. Multiplexer 305 selects imagery data from the first set of memory buffers based on a clock signal (e.g., pulse 312).

Otherwise, multiplexer MUX 305 selects imagery data from a first neighbor CNN processing engine (from the left side of FIG. 3 not shown) through a clock-skew circuit 320.

At the same time, a copy of the imagery data fed into the CNN processing block 304 is sent to a second neighbor CNN processing engine (to the right side of FIG. 3 not shown) via the clock-skew circuit 320. Clock-skew circuit 320 can be achieved with known techniques (e.g., a D flip-flop 322).

The first neighbor CNN processing engine may be referred to as an upstream neighbor CNN processing engine in the loop formed by the clock-skew circuit 320. The second neighbor CNN processing engine may be referred to as a downstream CNN processing engine. In another embodiment, when the data flow direction of the clock-skew circuit is reversed, the first and the second CNN processing engines are also reversed becoming downstream and upstream neighbors, respectively.

After 3×3 convolutions for each group of imagery data are performed for predefined number of filter coefficients, convolution operations results Out(m, n) are sent to the first set of memory buffers via another multiplex MUX 307 based on another clock signal (e.g., pulse 311). An example clock cycle 310 is drawn for demonstrating the time relationship between pulse 311 and pulse 312. As shown pulse 311 is one clock before pulse 312, as a result, the 3×3 convolution operations results are stored into the first set of memory buffers after a particular block of imagery data has been processed by all CNN processing engines through the clock-skew circuit 320.

After the convolution operations result Out(m, n) is obtained from Formula (1), rectification procedure may be performed as directed by image processing control 218. Any convolution operations result, Out(m, n), less than zero (i.e., negative value) is set to zero. In other words, only positive value of output results are kept. FIG. 8 shows two example outcomes of rectification. A positive output value 10.5 retains as 10.5 while −2.3 becomes 0. Rectification causes non-linearity in the integrated circuits.

If a 2×2 pooling operation is required, the M×M output results are reduced to (M/2)×(M/2). In order to store the (M/2)×(M/2) output results in corresponding locations in the first set of memory buffers, additional bookkeeping techniques are required to track proper memory addresses such that four (M/2)×(M/2) output results can be processed in one CNN processing engine.

To demonstrate a 2×2 pooling operation, FIG. 9A is a diagram graphically showing first example output results of a 2-pixel by 2-pixel block being reduced to a single value 10.5, which is the largest value of the four output results. The technique shown in FIG. 9A is referred to as “max pooling”. When the average value 4.6 of the four output results is used for the single value shown in FIG. 9B, it is referred to as “average pooling”. There are other pooling operations, for example, “mixed max average pooling” which is a combination of “max pooling” and “average pooling”. The main goal of the pooling operation is to reduce the size of the imagery data being processed. FIG. 10 is a diagram illustrating M×M pixel locations, through a 2×2 pooling operation, being reduced to (M/2)×(M/2) locations, which is one fourth of the original size.

An input image generally contains a large amount of imagery data. In order to perform image processing operations. The input image 1100 is partitioned into M-pixel by M-pixel blocks 1111-1112 as shown in FIG. 11A. Imagery data associated with each of these M-pixel by M-pixel blocks is then fed into respective CNN processing engines. At each of the M×M pixel locations in a particular M-pixel by M-pixel block, 3×3 convolutions are simultaneously performed in the corresponding CNN processing block.

Although the invention does not require specific characteristic dimension of an input image, the input image may be required to resize to fit into a predefined characteristic dimension for certain image processing procedures. In an embodiment, a square shape with (2^(K)×M)-pixel by (2^(K)×M)-pixel is required. K is a positive integer (e.g., 1, 2, 3, 4, etc.). When M equals 14 and K equals 4, the characteristic dimension is 224. In another embodiment, the input image is a rectangular shape with dimensions of (2^(I)×M)-pixel and (2^(J)×M)-pixel, where I and J are positive integers.

In order to properly perform 3×3 convolutions at pixel locations around the border of a M-pixel by M-pixel block, additional imagery data from neighboring blocks are required. FIG. 11B shows a typical M-pixel by M-pixel block 1120 (bordered with dotted lines) within a (M+2)-pixel by (M+2)-pixel region 1130. The (M+2)-pixel by (M+2)-pixel region is formed by a central portion of M-pixel by M-pixel from the current block, and four edges (i.e., top, right, bottom and left) and four corners (i.e., top-left, top-right, bottom-right and bottom-left) from corresponding neighboring blocks. Additional details are shown in FIG. 12 and corresponding descriptions for the first set of memory buffers.

FIG. 11C shows two example M-pixel by M-pixel blocks 1122-1124 and respective associated (M+2)-pixel by (M+2)-pixel regions 1132-1134. These two example blocks 1122-1124 are located along the perimeter of the input image. The first example M-pixel by M-pixel block 1122 is located at top-left corner, therefore, the first example block 1122 has neighbors for two edges and one corner. Value “0” s are used for the two edges and three corners without neighbors (shown as shaded area) in the associated (M+2)-pixel by (M+2)-pixel region 1132 for forming imagery data. Similarly, the associated (M+2)-pixel by (M+2)-pixel region 1134 of the second example block 1124 requires “0”s be used for the top edge and two top corners. Other blocks along the perimeter of the input image are treated similarly. In other words, for the purpose to perform 3×3 convolutions at each pixel of the input image, a layer of zeros (“0”s) is added outside of the perimeter of the input image. This can be achieved with many well-known techniques. For example, default values of the first set of memory buffers are set to zero. If no imagery data is filled in from the neighboring blocks, those edges and corners would contain zeros.

Furthermore, an input image can contain a large amount of imagery data, which may not be able to be fed into the CNN processing engines in its entirety. Therefore, the first set of memory buffers is configured on the respective CNN processing engines for storing a portion of the imagery data of the input image. The first set of memory buffers contains nine different data buffers graphically illustrated in FIG. 12. Nine buffers are designed to match the (M+2)-pixel by (M+2)-pixel region as follows:

1) buffer-0 for storing M×M pixels of imagery data representing the central portion;

2) buffer-1 for storing 1×M pixels of imagery data representing the top edge;

3) buffer-2 for storing M×1 pixels of imagery data representing the right edge;

4) buffer-3 for storing 1×M pixels of imagery data representing the bottom edge;

5) buffer-4 for storing M×1 pixels of imagery data representing the left edge;

6) buffer-5 for storing 1×1 pixels of imagery data representing the top left corner;

7) buffer-6 for storing 1×1 pixels of imagery data representing the top right corner;

8) buffer-7 for storing 1×1 pixels of imagery data representing the bottom right corner; and

9) buffer-8 for storing 1×1 pixels of imagery data representing the bottom left corner.

Imagery data received from the I/O data bus are in form of M×M pixels of imagery data in consecutive blocks. Each M×M pixels of imagery data is stored into buffer-0 of the current block. The left column of the received M×M pixels of imagery data is stored into buffer-2 of previous block, while the right column of the received M×M pixels of imagery data is stored into buffer-4 of next block. The top and the bottom rows and four corners of the received M×M pixels of imagery data are stored into respective buffers of corresponding blocks based on the geometry of the input image (e.g., FIGS. 11A-11C).

An example second set of memory buffers for storing filter coefficients are shown in FIG. 13A. In one embodiment, a pair of independent buffers Buffer0 1301 and Buffer1 1302 is provided. The pair of independent buffers allow one of the buffers 1301-1302 to receive data from the I/O data bus 1330 while the other one to feed data into a CNN processing block (not shown). Two operational modes are shown herein.

Example storage schemes of filter coefficients are shown in FIG. 13B. Each of the pair of buffers (i.e., Buffer0 1301 or Buffer1 1302) has a width (i.e., word size 1310). In one embodiment, the word size is 120-bit. Accordingly, each of the filter coefficients (i.e., C(3×3) and b) occupies 12-bit in the first example storage scheme 1311. In the second example storage scheme 1312, each filter coefficient occupies 6-bit thereby 20 coefficients are stored in each word. In the third example scheme 1313, 3-bit is used for each coefficient hence four sets of filter coefficients (40 coefficients) are stored. Finally, in the fourth example storage scheme 1314, 80 coefficients are stored in each word, each coefficient occupies 1.5-bit.

In another embodiment, a third memory buffer can be set up for storing entire filter coefficients to avoid I/O delay. In general, the input image must be at certain size such that all filter coefficients can be stored. This can be done by allocating some unused capacity in the first set of memory buffers to accommodate such a third memory buffer. Since all memory buffers are logically defined in RAM (Random-Access Memory), well known techniques may be used for creating the third memory buffer. In other words, the first and the second sets of memory buffers can be adjusted to fit different amounts of imagery data and/or filter coefficients. Furthermore, the total amount of RAM is dependent upon what is required in image processing operations.

When more than one CNN processing engine is configured on the integrated circuit. The CNN processing engine is connected to first and second neighbor CNN processing engines via a clock-skew circuit. For illustration simplicity, only CNN processing block and memory buffers for imagery data are shown. An example clock-skew circuit 1440 for a group of CNN processing engines are shown in FIG. 14. The CNN processing engines connected via the second example clock-skew circuit 1440 to form a loop. In other words, each CNN processing engine sends its own imagery data to a first neighbor and, at the same time, receives a second neighbor's imagery data. Clock-skew circuit 1440 can be achieved with well-known manners. For example, each CNN processing engine is connected with a D flip-flop 1442.

A special case with only two CNN processing engines are connected in a loop, the first neighbor and the second neighbor are the same.

Referring now to FIG. 15, it is a schematic diagram showing an example image processing technique based on convolutional neural networks in accordance with an embodiment of the invention. Based on convolutional neural networks, multi-layer input imagery data 1511 a-1511 c is processed with convolutions using a first set of filters or weights 1520. Since the imagery data 1511 a-1511 c is larger than the filters 1520. Each corresponding overlapped sub-region 1515 of the imagery data is processed. After the convolutional results are obtained, activation may be conducted before a first pooling operation 1530. In one embodiment, activation is achieved with rectification performed in a rectified linear unit (ReLU). As a result of the first pooling operation 1530, the imagery data is reduced to a reduced set of imagery data 1531 a-1531 c. For 2×2 pooling, the reduced set of imagery data is reduced by a factor of 4 from the previous set.

The previous convolution-to-pooling procedure is repeated. The reduced set of imagery data 1531 a-1531 c is then processed with convolutions using a second set of filters 1540. Similarly, each overlapped sub-region 1535 is processed. Another activation can be conducted before a second pooling operation 1540. The convolution-to-pooling procedures are repeated for several layers and finally connected to a Fully-connected Networks (FCN) 1560. In image classification, respective probabilities of predefined categories can be computed in FCN 1560.

This repeated convolution-to-pooling procedure is trained using a known dataset or database. For image classification, the dataset contains the predefined categories. A particular set of filters, activation and pooling can be tuned and obtained before use for classifying an imagery data, for example, a specific combination of filter types, number of filters, order of filters, pooling types, and/or when to perform activation. In one embodiment, convolutional neural networks are based on Visual Geometry Group (VGG16) architecture neural nets, which contains 13 convolutional layers and three fully-connected network layers.

A trained convolutional neural networks model is achieved with an example set of operations 1600 shown in FIG. 16. At action 1602, a convolutional neural networks model is first obtained by training the convolutional neural networks model based on image classification of a labeled dataset, which contains a sufficiently large number of input data (e.g., imagery data, converted voice data, optical character reorganization (OCR) data, etc.). For example, there are at least 4,000 data for each category. In other words, each data in the labeled dataset is associated with a category to be classified. The convolutional neural networks model includes multiple ordered filter groups (e.g., each filter group corresponds to a convolutional layer in the convolutional neural networks model). Each filter in the multiple ordered filter groups contains a standard 3×3 filter kernel (i.e., nine coefficients in floating point number format (e.g., standard 3×3 filter kernel 1710 in FIG. 17)). Each of the nine coefficients can be any negative or positive real number (i.e., a number with fraction). The initial convolutional neural networks model may be obtained from many different frameworks including, but not limited to, Mxnet, caffe, tensorflow, etc.

Then, at action 1604, the convolutional neural networks model is modified by converting respective standard 3×3 filter kernels 1710 to corresponding bi-valued 3×3 filter kernels 1720 of a currently-processed filter group in the multiple ordered filter groups based on a set of kernel conversion schemes. In one embodiment, each of the nine coefficients C(i,j) in the corresponding bi-valued 3×3 filter kernel 1720 is assigned a value ‘A’ equal to the average of absolute coefficient values multiplied by the sign of corresponding coefficients in the standard 3×3 filter kernel 1710 shown in following formula:

$\begin{matrix} {A = {\sum\limits_{{1 \leq i},{j \leq 3}}{{{C\left( {i,j} \right)}}/9}}} & (2) \end{matrix}$

Filter groups are converted one at a time in the order defined in the multiple ordered filter groups. In certain situation, two consecutive filter groups are optionally combined such that the training of the convolutional neural networks model is more efficient.

Next, at action 1606, the modified convolutional neural networks model is retrained until a desired convergence criterion is met or achieved. There are a number of well known convergence criteria including, but not limited to, completing a predefined number of retraining operation, converging of accuracy loss due to filter kernel conversion, etc. In one embodiment, all filter groups including already converted in previous retraining operations can be changed or altered for fine tuning. In another embodiment, the already converted filter groups are frozen or unaltered during the retraining operation of the currently-processed filter group.

Process 1600 moves to decision 1608, it is determined whether there is another unconverted filter group. If ‘yes’, process 1600 moves back to repeat actions 1604-1606 until all filter groups have been converted. Decision 1608 becomes ‘no’ thereafter. At action 1610, coefficients of bi-valued 3×3 filter kernels in all filter groups are transformed from a floating point number format to a fixed point number format to accommodate the data structure required in the CNN based integrated circuit. Furthermore, the fixed point number is implemented as reconfigurable circuits in the CNN based integrated circuit. In one embodiment, the coefficients are implemented using 12-bit fixed point number format.

FIG. 18 is a diagram showing an example data conversion scheme for converting data from 8-bit [0-255] to 5-bit [0-31] per pixel. For example, bits 0-7 becomes 0, bits 8-15 becomes 1, etc.

Referring now to FIG. 19A, it is shown data flow of an example feature extractor 1920 in accordance with one embodiment of the invention. Input data or imagery data 1910 are fed into a feature extractor 1920 to produce a set of extracted features 1930. Example input data or imagery data 1910 include, but are not limited to, still photo, picture, a frame of a video stream, converted form of voice data, optical character recognition (OCR), etc. Feature extractor 1920 is a CNN based integrated circuit (e.g., CNN based IC 100 of FIGS. 1A-1B) with a set of filter coefficients or weights configured thereon. The set of filter coefficients or weights are pre-trained for a specific domain (e.g., image, converted voice spectrum, etc.). Pre-trained filter coefficients are loaded into the CNN based integrated circuit entirely or in-part depending upon the amount and size of the filter coefficients. Extracted features 1930 are results of performing a number of convolutional operations along with activation and pooling operations over the input data 1920 in the feature extractor 1920.

As described in process 1600 of FIG. 16, a convolutional neural networks model is trained for the CNN based integrated circuit. The entire set of trained coefficients or weights are pre-configured to the CNN based integrated circuit as a feature extractor for a particular data format (e.g., imagery data, converted voice data, optical character recognition (OCR), etc.). In general, there are many convolutional layers with many filters in each layer. In one embodiment, VGG16 model contains 13 convolutional layers. In a software based image classification task, computations for the convolutional layers take majority of computations (e.g., 90%) traditionally. This computations is drastically reduced with a dedicated hardware such as CNN based IC 100.

Input data is then classified in a predefined set of categories in a hierarchical category classification scheme using multiple fully-connected networks (FCNs), which are pre-trained for various level of categories.

Each FCN is capable of classifying the input data via the extracted features in a specific level of the hierarchical category classification scheme. First, a root level FCN is used for classifying the input data among a set of top level categories. Then, a relevant next level FCN is used in conjunction with the same extracted features for further classifying the input data among a set of subcategories to the category determined in previous level. Hierarchical category classification scheme continues for further detailed subcategories if desired

FIGS. 19B-19D are schematic diagrams showing an example hierarchical classification scheme with multiple fully-connected networks in accordance with one embodiment of the invention.

First, extracted features 1930 are processed through a root level FCN (i.e., Model-L0 1940). The results are respective probabilities 1960 of a set of top level categories 1950 in the first level of classification of the hierarchical category classification scheme. As shown in this example in FIG. 19B, the top level categories are “Animal”, “Car”, and so on. The input data is found to have 90.2% probability to be Category “Animal”.

Next, the same extracted features 1930 are processed through a relevant next level FCN (i.e., Model-L1-Animal 1941). The results are again respective probabilities 1961 of a set of subcategories 1951 to Category “Animal”. In this example shown in FIG. 19C, subcategories are “Dog”, “Cat”, “Horse”, etc. In this level, the input data is found to have 88.8% probability to be Category “Dog”.

Then, the same extracted features 1930 are processed through a another relevant next level FCN (i.e., Model-L2-Dog 1942). Probabilities 1962 of the more detailed subcategories 1952 to Category “Dog” are resulted. It can be seen that the more detailed subcategories for “Dog” are “Poodle”, “Bulldog”, . . . , “Golden Retriever”. In this example shown in FIG. 19D, the input image is found 98.5% probability to be “Golden Retriever”.

Using hierarchical category classification scheme, each FCN is responsible for classifying input data among a relatively smaller number of categories thereby more efficient and accurate. And training of an image classification model becomes relatively easier because only few layers in FCN need to be adjusted.

FIG. 20 is a schematic diagram showing example connection patterns an example FCN in accordance with one embodiment of the invention. Extracted features 2030 and a set of categories 2050 are fully-connected in an example FCN, which is shown as more than one layers (e.g., FCN Layer-1 2040 a, FCN Layer-2 2040 b, etc.).

A weight or coefficient is associated with each connection between a feature in the extracted features 2030 and a cell or neuron of the FCN layer-1 2040 a. Similarly a weight is associated with each and every connections between cells of the layers in FCN. For illustration simplicity and clarity, only few features, cells and categories are drawn and shown in FIG. 20. In order to obtain respective probabilities of the set of categories, many computations are required as each value in a neuron is multiplied with respective weights for each level of the FCN. As the feature extraction portion is sped up drastically with the dedicated hardware (i.e., CNN based IC 100), the computations in FCN become more dominated. Techniques to computation speed in the FCN portion becomes important.

One of the techniques is to reduce the number of computations based on the numerical value of weights obtained in training of a fully-connected networks. FIG. 21A is a schematic diagram showing connection patterns between FCN Layer-A 2121 and FCN Layer-B 2122. FCN Layer-A 2121 contains I cells or neurons while FCN Layer-B 2122 contains J cells or neurons. To fully connect these two layers, there are the total number of connections equal to I×J. Each connection is associated with a weight. For example, the first neuron or cell of FCN Layer-A 2121 connects to the first neuron or cell of FCN Layer-B 2122 via a connection having weight W₁₁. The second cell of FCN Layer-A 2121 to the first cell of FCN Layer-B 2122 is associated with weight W₂₁. For a generic connection, the corresponding weight W_(ij) with i in [1,I] and j in [1,J]. In one embodiment, I and J are both large numbers, for example, I is larger than 4096 while J is 4096. For layer connecting the feature extractor and FCN, I can be 25,088 and I is 4096.

To reduce the number of computations, the connection is removed when the numerical value of the corresponding weight or coefficient is equal to zero or less than a predefined threshold value. FIG. 21B shows such an example for W₁₁=0 and W_(2J) is less than a threshold value V_(th). Due to huge number of connections existed between two FCN layers 2121-2122, only few representative connections are shown in FIG. 21B for simplicity and clarity. For those having ordinary skill in the art would understand fully-connected networks are too large to shown in a schematic diagram.

Another computation reduction scheme is to remove those neurons or cells in a FCN layer that do not provide any contribution to the output results (i.e., those neurons or cells having numerical value of zero). FIG. 23 is a flowchart illustrating an example process 2300 of the computation reduction scheme in a FCN.

Process 2300 starts at action 2302 to obtain input values in each layer of a FCN by running all data in a desired dataset (e.g., labeled dataset for training). Then, at action 2304, those neurons that have numerical value of zero at least certain threshold percentage of times are identified. Finally, at action 2306, the identified neurons to reduce the size of the FCN layers are removed thereby compressing the FCN model. The threshold percentage can be empirically-determined with each different dataset, for example, 90% for a particular dataset in a specific domain.

A second example computation reduction scheme is shown as schematic diagrams in FIGS. 22A-22B. Neurons or cells of FCN Layer-C 2221 are fully-connected with neurons of cells of FCN Layer-D 2222 in FIG. 22A. Any neuron or cell has a numerical value of zero is checked to see if it is zero 2231-2232 for at least certain threshold percentage of times for all data in a desired dataset (i.e., training dataset for a particular category). If so, the corresponding neuron or cell is removed in the FCN as shown in FIG. 22B and the connections are reduced. As the number of neurons or cells are huge numbers in each layer of a FCN, the reduction of these non-contributing neurons/cells can improve the computational speed and still maintain accuracy.

For better extracting features in different domains, like speech, face recognition, gesture recognition and etc, different sets of configured convolution layer coefficients are provided for that domain. And the particular set of convolution layers is used as a general feature extractor for the specific tasks in that domain. For example, the specific task of family members face recognition in the domain of face recognition, and the specific task of company employee face recognition also in the same domain of face recognition. And these two specific tasks can share the same set of convolution layers coefficients used for face detection but using different FCNs.

For different application domains, a set of pre-trained convolution coefficients based on large dataset is provided. For example, for image domain, a set of pre-trained ImageNet bi-value model is provided and loaded into the CNN based IC, and this convolution coefficients may be used in 1000-categories image related area. Based on that, users only use fixed convolution coefficients to train FCNs only. For voice domain, a set of pre-trained convolution coefficients based on voice spectrum is provided.

Although the invention has been described with reference to specific embodiments thereof, these embodiments are merely illustrative, and not restrictive of, the invention. Various modifications or changes to the specifically disclosed example embodiments will be suggested to persons skilled in the art. For example, whereas the input image has been shown and described as partitioning into M-pixel by M-pixel blocks in certain order, other orders may be used in the invention to achieve the same, for example, the ordering of the M-pixel by M-pixel blocks may be column-wise instead of row-wise. Furthermore, whereas M-pixel by M-pixel blocks have been shown and described using M equals to 14 as an example. M can be chosen as other positive integers to accomplish the same, for example, 16, 20, 30, etc. Additionally, whereas the 3×3 convolution and 2×2 pooling have been shown and described, other types of convolution and pooling operations may be used to achieve the same, for example, 5×5 convolution and 3×3 pooling. Further, whereas hierarchical category classification scheme has been shown and described in a three-level hierarchy example, the invention does not set limit as to how many levels can be employed in the scheme, for example, four, five or even more levels of hierarchy. In summary, the scope of the invention should not be restricted to the specific example embodiments disclosed herein, and all modifications that are readily suggested to those of ordinary skill in the art should be included within the spirit and purview of this application and scope of the appended claims. 

What is claimed is:
 1. A system for classifying input data using a hierarchical category classification scheme comprising: a cellular neural networks (CNN) based integrated circuit being loaded with pre-trained filter coefficients of convolutional layers for extracting features out of an input data that belong to a particular domain; and a multi-processor computing unit configured for using multiple hierarchically-ordered groups of pre-trained fully-connected networks (FCNs) in a hierarchical category classification scheme that contains a set of top level categories and each of the top level categories contains at least one set of subcategories, the extracted features being repeatedly processed through corresponding ones of the multiple hierarchically-ordered groups to identify the input data as a most probable category, the multiple hierarchically-ordered groups of FCNs containing a root level group configured for the set of top level categories and at least one next level group configured for the at least one set of subcategories.
 2. The system of claim 1, wherein the CNN based integrated circuit comprises a plurality of cellular neural networks (CNN) processing engines operatively coupled to the at least one input/output data bus, the plurality of CNN processing engines being connected in a loop with a clock-skew circuit, each CNN processing engine comprising: a CNN processing block configured for simultaneously obtaining convolution operations results using the input data and the pre-trained filter coefficients; a first set of memory buffers operatively coupling to the CNN processing block for storing the input data to be fed into the CNN block; and a second set of memory buffers operative coupling to the CNN processing block for storing the pre-trained filter coefficients to be fed into the CNN processing block.
 3. The system of claim 1, wherein the particular domain comprises converted voice spectrum data.
 4. The system of claim 1, wherein the particular domain comprises imagery data for face recognition.
 5. The system of claim 1, wherein the particular domain comprises imagery data for optical character recognition.
 6. The system of claim 1, wherein the particular domain comprises converted voice spectrum data for voice recognition.
 7. The system of claim 1, wherein the root level group is used for classifying the input data by identify the input data as the most probable category among the set of top level categories.
 8. The system of claim 1, wherein a relevant one of the at least one next level group is used for classifying the input data among a corresponding set of the at least one set of subcategories.
 9. The system of claim 1, wherein each of the multiple hierarchically-ordered groups contains at least one FCN layer with each FCN layer containing multiple neurons or cells.
 10. The system of claim 9, wherein said each FCN layer is reduced in size by removing those neurons or cells have input value of zero for more than a threshold percentage of all data in a desired dataset.
 11. The system of claim 10, wherein the desired dataset comprises a labeled dataset for training in the particular domain.
 12. The system of claim 10, wherein the threshold percentage is determined from the desired training dataset.
 13. The system of claim 10, wherein the threshold percentage is 90%.
 14. The system of claim 9, wherein said each FCN layer is reduced in size by removing those connections when numerical value of corresponding weights is equal to zero.
 15. The system of claim 9, wherein said each FCN layer is reduced in size by removing those connections when numerical value of corresponding weights is less than a threshold value.
 16. The system of claim 15, wherein the threshold value is determined that the contribution of the connection is too small for corresponding feature represented by the neuron.
 17. The system of claim 1, wherein the pre-trained filter coefficients of convolutional layers comprises bi-valued 3×3 filter coefficients or weights.
 18. The system of claim 1, wherein the hierarchical category classification scheme is derived from Visual Geometry Group's VGG16 model with 13 convolutional layers and three FCN layers.
 19. The system of claim 17, wherein the pre-trained filter coefficients of convolutional layers are used unaltered in conjunction with each group of the multiple hierarchically-ordered groups of FCNs. 